The present document is based on Japanese Priority Document JP 2001-278192, filed in the Japanese Patent Office on Sep. 13, 2001, the entire contents of which being incorporated herein by reference.
1. Field of the Invention
The present invention particularly relates to a high density magnetic recording medium which is applicable to an MR (magneto-resistive) head and a GMR (giant magneto-resistive) head.
2. Description of Related Art
Conventionally, as a magnetic recording medium, a so-called coating type magnetic recording medium is well-known, which is manufactured in such a way that a magnetic coating where a magnetic oxide powder or a magnetic alloy powder is dispersed in an organic binder such as polyvinylchrolide polyvinylacetate co-polymer, polyester resin, polyurethane resin is coated on a non-magnetic base and then dried.
While, according to a need for higher density recording, a magnetic recording medium having a magnetic layer comprising a ferromagnetic thin film where a ferromagnetic material comprising a metal or an alloy such as Coxe2x80x94Ni is directly coated on a non-magnetic base by means of plating or a vacuum thin-film forming process (such as a vacuum deposition process, a sputtering process, an ion plating process, and the like) is used in practice.
Such a magnetic recording medium of a so-called magnetic thin metal film type has several advantages in coercive force, remanent magnetization, squareness ratio, and not only has an excellent electromagnetic transformation performance at a shorter wavelength but also may be formed to have a very thin magnetic layer so that a demagnetization in records and a caliper loss during reproduction may be reduced. Further, since it is not necessary to mix a binder of a non-magnetic material into the magnetic layer, the medium may have a high filling up ratio of a magnetic material so as to obtain a higher magnetization, for example.
Further, in an effort to obtain a larger output by improving an electromagnetic transformation performance in such a kind of magnetic recording medium, a so-called oblique evaporation a magnetic layer is obliquely or tiltedly formed by means a vapor deposition process has been proposed and used in practice in a magnetic tape for a high quality VTR, a digital VTR, a data storage, and the like.
Although the magnetic recording medium of magnetic metal thin-film type has advantages as described above, there is a need for forming fine projections on an outermost surface on the magnetic layer side in order to reduce friction between a recording/reproducing magnetic head and a slide-contact surface of the magnetic recording mediumxe2x80x94that is the outermost surface of the magnetic layer sidexe2x80x94and to obtain sufficient durability of the magnetic layer.
In particular, according to a need for higher density recording, it is required that the fine projections should have an even and appropriate height in order to reduce a spacing loss and be dispersed in the magnetic layer evenly and in an appropriate density in order to obtain an appropriate electromagnetic transformation performance while maintaining low friction and high durability as well, and the magnetic layer is required to be formed much thinner because of a need for a smaller medium with a larger capacity.
Further, in response to a higher recording density in the magnetic recording medium, it is expected that, as to a magnetic head, a conventional inductive head will be replaced with a more sensitive head such as a magneto-resistive head (MR head) and a giant magneto-resistive head (GMR head).
When a magnetic head of a magneto-resistive effect type such as an MR head and a GMR head is used for a magnetic recording medium of a thin metal film type, the magnetic head provides a higher output than the conventional inductive head and has advantages in that the output does not depend on a relative speed between the medium tape and the head, for example.
However, a conventional metal evaporated tape generates an excessive amount of magnetic flux so that a resistance change in an MR element is out of a linear area. As a result it becomes difficult to obtain a distortion-free characteristic.
Further, a reproducing head has a high sensitivity so that a media noise becomes dominant in a C/Nmedia (C/N: carrier to noise ratio), whereby a conventional surface design on the magnetic layer side causes an excessive amount of noises.
In addition, an abrasion of the MR element causes a problem in that a reproduced waveform becomes asymmetrical, for example.
In consideration of the above, as for a magnetic recording medium applicable to a highly sensitive magnetic head such as an MR head and a GMR head, the present inventors have made an effort to provide a magnetic recording medium capable of reducing a media noise and an abrasion in a reproducing MR head or a reproducing GMR head and of having durability as well by optimizing a thickness of the magnetic layer and a magnetic characteristic and further of controlling the fine projections on the surface of the magnetic layer.
A magnetic recording medium according to the present invention, comprises a magnetic layer, on a main surface of a non-magnetic base, which is formed by means of a thin-film forming technology by vacuum, and the magnetic recording medium is applicable to a reproducing system using a magneto-resistance effect type (magneto-resistive) head (MR head) or a giant magneto-resistance effect type (giant magneto-resistive) head (GMR head). Fine projections are formed at the outermost surface of the magnetic layer side.
Assuming that a height of the fine projections is h and a diameter of the fine projections at a half-height h/2 is xcfx86h/2, 99% of the fine projections having a height not less than 10 nm satisfy a relationship expressed by 100 nmxe2x89xa7xcfx86h/2xe2x89xa740 nm, and a relationship between the height h of the fine projections and a density P (a number of fine projections per square mm) of the fine projections is as follows:
for h greater than 25 nm, P is 250,000/mm2 or less;
for 25 nmxe2x89xa7h greater than 20 nm, P is 300,000/mm2 or more and 1,250,000/mm2 or less;
for 20 nmxe2x89xa7h greater than 15 nm, P is 1,000,000/mm2 or more and 3,750,000/mm2 or less;
for 15 nmxe2x89xa7h greater than 10 nm, P is 8,000,000/mm2 or more and 25,000,000/mm2 or less; and
for 10 nmxe2x89xa7h greater than 5 nm, P is 50,000,000/mm2 or less.
By controlling the conditions as described above, the present invention provides a magnetic recording medium which reduces noises, obtains a high C/N, reduces abrasion in the reproducing MR head and the reproducing GMR head, and also has good durability.
By controlling fine projections on the surface of the non-magnetic base, a height and a density of fine projections on the magnetic layer, the C/Nmedia may be improved, good shuttling durability and corrosion resistance may be provided, and abrasion resistance of the MR head may be improved.
A magnetic recording medium according to the present invention, comprises a magnetic layer having a thickness of 30 nm or more and 80 nm or less on a main surface of a non-magnetic base. The magnetic layer is fabricated by means of a thin-film forming technology by vacuum, and the magnetic recording medium is a magnetic tape applicable to a helical scanning recording/reproducing system using a reproducing MR head or a reproducing GMR head; and fine projections are formed at the outermost surface of the magnetic layer side.
Assuming that the height of the fine projections is h and the diameter of the fine projections at the half-height h/2 is xcfx86h/2, 99% of the fine projections having a height not less than 10 nm are controlled to satisfy a relationship as shown in a formula 100 nmxe2x89xa7xcfx86h/2xe2x89xa740 nm, and a relationship between a height h of the fine projections and a density P of the fine projections as follows:
for h greater than 25 nm, P is 250,000/mm2 or less;
for 25 nmxe2x89xa7h greater than 20 nm, P is 300,000/mm2 or more and 1,250,000/mm2 or less;
for 20 nmxe2x89xa7h greater than 15 nm, P is 1,000,000/mm2 or more and 3,750,000/mm2 or less;
for 15 nmxe2x89xa7h greater than 10 nm, P is 8,000,000/mm2 or more and 25,000,000/mm2 or less; and
for 10 nmxe2x89xa7h greater than 5 nm, P is 50,000,000/mm2 or less.
An embodiment of the magnetic recording medium according to the present invention will be described with reference to tables in attached figures, however, the present invention is not limited to the embodiment or examples.
The non-magnetic base may be any of a polyethylene terephthalate film, a polyethylene naphthalate film, a polyimide film, a polyamide film, a poly (ether imide) film, etc.
Surface properties of the non-magnetic base affects surface properties of the magnetic layer when the magnetic layer is formed by means of a vacuum thin-film forming process, and also affects a C/Nmedia, runnability, and durability of a finished magnetic recording medium, therefore it is necessary to control the surface properties of the non-magnetic base.
In order to obtain a high C/Nmedia, a magnetic recording medium with the least possible number of projections may be chosen to even the surface of the magnetic layer. However, if the magnetic layer is too even, friction between the magnetic layer and the head becomes large, and therefore runnability and durability for a magnetic recording medium are degraded. Whereas, if more projections are formed, a higher durability may be obtained, however, it is difficult to realize a high C/Nmedia, thereby causing an abrasion of the reproducing head and making a reproduced waveform asymmetrical. Thus, by controlling projections formed on the surface of the non-magnetic base, a height and a density of fine projections formed on the surface of the magnetic layer of the magnetic recording medium were controlled.
Magnetic metal materials for forming the magnetic layer may be any material as far as it is used for a conventional metal evaporated tape. For example, a ferromagnetic metals such as Fe, Co, Ni and a ferromagnetic alloy such as Fexe2x80x94Co, Coxe2x80x94Ni, Coxe2x80x94Cr, Coxe2x80x94Crxe2x80x94Ta, Coxe2x80x94Crxe2x80x94Pt, Coxe2x80x94Nixe2x80x94Pt, Fexe2x80x94Coxe2x80x94Ni, Fexe2x80x94Coxe2x80x94B, Fexe2x80x94Nixe2x80x94B, Fexe2x80x94Coxe2x80x94Nixe2x80x94Cr may be selected.
The magnetic layer may be formed into a thin-film by means of PVD (physical vapor deposition) technology such as a vapor-deposition process where a magnetic metal material is heated to evaporate and then deposited on the non-magnetic base, an ion plating process where evaporation of a magnetic metal material is carried out while discharging, a sputtering process where glow discharging is caused in an atmosphere mainly containing argon and an argon ion hits and drives an atom on a target surface into the atmosphere.
Here, the magnetic layer may be a single layer film or a multi-layer film which are formed by means of the above process. Further, an underlying layer or an intermediate layer may be disposed between the non-magnetic base and the magnetic layer, or between multiple layers for a magnetic layer having such multiple layers in order to improve adhesion between the multiple layers and to control coercive force. In addition, an oxide may be adopted in the vicinity of the surface of the magnetic layer in order to improve its corrosion resistance, for example.
It is preferred that a protective layer is formed on the magnetic layer of the magnetic recording medium and the protective layer is mainly formed of carbon in order to improve the durability and the corrosion resistance.
The protective layer may be formed by means of a conventional vacuum film-forming technology. For example, a CVD (chemical vapor deposition) process where a carbon compound is decomposed in a plasma atmosphere and deposited on a magnetic layer may steadily form a film of a hard carbon referred to as a diamond-like carbon which has advantages in abrasion resistance, corrosion resistance, and surface coverage, also has an even surface form and a high electric resistance.
As to carbon compounds used for forming the protective layer, any conventional material such as hydrocarbons, ketones, and alcohols may be used. Further, while generating plasma, Ar and H2 may be induced as a gas for promoting decomposition of hydrocarbons. Still further, in order to improve film hardness and corrosion resistance of the diamond-like carbon, the carbon may have been reacted with nitrogen and fluorine, and the diamond-like carbon layer may be a single layer or multiple layers. In addition, while generating plasma, gases such as N2, CHF3, and CH2F2 in addition to hydrocarbons may be used alone or in a state of an appropriate mixture so as to form a film.
If the protective layer is formed too thick, a spacing loss increases, whereas, if it is too thin, durability and corrosion resistance drop. Thus, the protective layer is preferably formed in a thickness between about 4 nm and about 15 nm.
The magnetic recording medium according to the present invention has a back coat layer of a predetermined material on the other side of the surface where the magnetic layer is formed, and the back coat layer preferably has a thickness between 0.3 xcexcm and 0.8 xcexcm. A thickness of less than 0.3 xcexcm does not effect as a back coat layer. A thickness of more than 0.8 xcexcm increases the total thickness of the medium. As a result, the medium of a specified length cannot be accommodated within a cassette casing. A predetermined lubricant may be applied to both surface layers so as to obtain sufficient runnability and durability.
The magnetic recording medium having the construction as described above satisfies, assuming that the height of the fine projections on the outermost surface on the surface where the magnetic layer is formed is h and the diameter of the fine projections at a half-height h/2 is xcfx86h/2, in 99% of the fine projections having a height not less than 10 nm, a relationship as shown in a formula 100 nmxe2x89xa7xcfx86h/2xe2x89xa740 nm, and the height h of the fine projections and a density P of the fine projections are specified as follows:
for h greater than 25 nm, P is 250,000/mm2 or less;
for 25 nmxe2x89xa7h greater than 20 nm, P is 300,000/mm2 or more and 1,250,000/mm2 or less;
for 20 nmxe2x89xa7h greater than 15 nm, P is 1,000,000/mm2 or more and 3,750,000/mm2 or less;
for 15 nmxe2x89xa7h greater than 10 nm, P is 8,000,000/mm2 or more and 25,000,000/mm2 or less; and
for 10 nmxe2x89xa7h greater than 5 nm, P is 50,000,000/mm2 or less.
In order to satisfy the above relationships about the outermost surface of the magnetic recording medium, it is effective to define the surface properties of the non-magnetic base itself and to control the film thickness of the magnetic layer at the same time. In other words, when the diameter of surface projections on the non-magnetic base having a thickness of 3.8 xcexcm or more and 5.3 xcexcm or less is reduced, assuming that, for projections on the surface of the non-magnetic base where the magnetic layer is not adhered, their height is hxe2x80x2 and a diameter of the projections at a half-height hxe2x80x2/2 is xcfx86xe2x80x2hxe2x80x2/2, 99% of the fine projections having a height greater than 10 nm on the surface of the non-magnetic base satisfy a relationship of 50 nmxe2x89xa7xcfx86xe2x80x2hxe2x80x2/2xe2x89xa710 nm.
A relationship between the height hxe2x80x2 of the fine projections and the density Pxe2x80x2 of the fine projections is as follows:
for hxe2x80x2 greater than 20 nm, Pxe2x80x2 is not greater than 100,000/mm2;
for 20 nmxe2x89xa7hxe2x80x2 greater than 15 nm, the density Pxe2x80x2 is between 100,000 /mm2 and 300,000/mm2 (inclusive);
for 15 nmxe2x89xa7hxe2x80x2 greater than 10 nm, Pxe2x80x2 is between 2,000,000/mm2 and 15,000,000/mm2 inclusive); and
for 10 nmxe2x89xa7hxe2x80x2 greater than 5 nm, Pxe2x80x2 is not greater than 30,000,000/mm2.
According to these conditions, the non-magnetic base is specified and the magnetic layer is formed so as to be not less than 30 nm and not greater than 80 nm in thickness.
Then, the magnetic layer is characterized in that an induced oxygen quantity, an electron beam power, and an adhering angle of an evaporated particle are adjusted so that a coercive force at 20xc2x0 C. is not less than 80 KA/m and not greater than 180 KA/m and a product Mrxc2x7t of remanent magnetization Mr and the film thickness is not less than 10 mA and not more that 30 mA. When the magnetic surface side of the magnetic recording medium is observed by means of an AFM (atomic force microscope), the magnetic layer has a surface where its arithmetic mean roughness Ra is 1.0 nm to 2.3 nm, the maximum roughness depth Ry is 20 nm to 50 nm, and mean roughness depth Rz is 15 nm to 35 nm.
As described above, it is very important to control the film thickness and magnetic characteristics when using a reproducing head of a magneto-resistance effect (magneto-resistive) type. Further, when the magnetic layer is formed thin, the magnetic recording medium does not provide a sufficient output and degrades its corrosion resistance. When the magnetic layer is formed thick, surface projections of the non-magnetic base do not affect the magnetic layer in terms of shape, which may cause an increase in friction and a decrease in durability.